Excluding an image plane from interacting with light
If I understand you correctly, you want the background plane to be unshaded. You could use a material that does not use a shader node, by creating a new material, removing the Principled BSDF node, then connecting a node like RGB or a texture node to the Surface output. Note, this would make it so that it would not interact with any light source. If you want it to interact with a light source, you would have to use Layers (Top Right) and then merge them together using the compositor and nodes. You can choose which things interact in layers using collections, by disabling what you don't want to affect the objects in your scene. Good luck!